The printing industry is constantly looking for printing techniques which produce printed matter which offers additional security or which is visually appealing in various applications.
In the security printing industry, printed matter which exhibits an effect when visually inspected under various light conditions, but which is not capable of replication using known duplicating methods, such as photocopying or scanning, is highly advantageous.
A security document having some of these features is disclosed in U.S. Pat. No. 4,420,515—Amon et al, which includes a metallic film which is printed or embossed to produce a latent image which is viewed to verify the authenticity of the document. However, the process by which this document is produced requires a number of complex steps to apply the metallic film to the substrate before printing and embossing, and since it would be impractical to incorporate these application steps in the usual document printing process, difficulties will be experienced in making the process commercially attractive. Furthermore, the image produced by the printing and embossing of the metallic film is a latent image which may require specific conditions for viewing and verification.
The introduction of banknotes printed on polymer substrates has introduced a further dimension to the security printing industry, and the present invention seeks to provide a further improvement in banknotes and other security devices exhibiting the abovementioned desirable effect.